|Veröffentlichungsdatum||13. Dez. 2016|
|Eingetragen||12. Nov. 2012|
|Prioritätsdatum||10. Nov. 2011|
|Auch veröffentlicht unter||US20130119987|
|Veröffentlichungsnummer||13674654, 674654, US 9519037 B2, US 9519037B2, US-B2-9519037, US9519037 B2, US9519037B2|
|Erfinder||Joel P. Felmlee|
|Ursprünglich Bevollmächtigter||Mayo Foundation For Medical Education And Research|
|Zitat exportieren||BiBTeX, EndNote, RefMan|
|Patentzitate (40), Nichtpatentzitate (6), Klassifizierungen (8), Juristische Ereignisse (1)|
|Externe Links: USPTO, USPTO-Zuordnung, Espacenet|
This application claims the benefit of U.S. Provisional patent application Ser. No. 61/558,100 filed on Nov. 10, 2011 and entitled “Spatially Coincident MRI Receiver Coils and Method for Manufacturing”.
The present invention generally relates to radio frequency (RF) receiver coils for magnetic resonance applications.
Magnetic resonance imaging (MRI) detects the faint nuclear magnetic resonance (NMR) signals given off by protons or other nuclei in the presence of a strong magnetic field after excitation with a RF signal. The NMR signals are detected using loop antennas termed “coils” or resonators.
Coils for magnetic resonance applications are typically operated at the Larmor frequency. The Larmor frequency depends on the strength of the basic magnetic field of the magnetic resonance system and on the chemical element whose excited spin is being detected. For hydrogen (which is the most frequent case), the gyromagnetic ratio is approximately 42.57 MHz/Tesla (1 Tesla=10000 gaus). During operation of a coil at resonance, a current oscillates with a resonance frequency in the conductor element. This current is particularly high when the conductor element is tuned to the resonance frequency. Thus, in the ideal case, the Larmor frequency corresponds to the resonance frequency of the resonator or coil.
NMR signals are, relatively speaking, extremely faint and therefore “local coils” or “surface coils” may be designed to be placed in close proximity to the region of interest of the imaged object. The size of the local coils is kept small to allow them to be easily fit to the patient on the MRI patient table. Importantly, the small area of loops of the local coil provides improved signal strength relative to received noise. The local coils are in contrast to the whole body coil typically present in an MRI machine and useful for obtaining broad survey scans of the patient.
The small size of a local coil generally limits the volume over which the coil is sensitive. For imaging large areas of the body, for example, neurovascular imaging of the head, neck, and lower spine, the whole body coil with its lower signal to noise ratio (SNR) must be used. Alternatively, coverage of this region can be obtained by using several local coils, taking multiple images of the patient and changing or repositioning the local coil in between images. This latter approach is time consuming and impractical in many situations. Additionally, these approaches may still suffer from lower SNR than is desired for imaging.
Mutli-layer conductors fashioned as resonators or coils for magnetic resonance imaging are also known, and examples include U.S. Pat. Nos. 7,579,835 and 7,579,836. However, as with single element coils, the use of multi-layer conductors in an MR coil does not, in itself, lead to improvements in SNR and uniformity over a range of clinically relevant loading conditions. In addition, when attempting to maintain SNR between subjects that present different loading conditions, a single coil would need to be re-tuned for each subject, or a number of single coils that were each tuned to different loading conditions would be needed. Alternatively, a broadband coil could be used to provide greater uniformity across loading conditions, but these broadband coils suffer from substantially lower SNR than could be achieved with single coils.
The present invention is directed generally to a coil for magnetic resonance applications that includes layers, or stacks, of independent conductive elements to provide high SNR and high uniformity. The additional layers in the stacked coil configuration achieve greater SNR and uniformity than a single layer of the same type could achieve on its own; especially when the variable loading conditions routinely found in clinical imaging are considered.
To this end, a coil, is provided that includes a conductor formed to have multiple layers that are electrically insulated from each other. The conductor is designed so that, during operation of the conductor at the resonance frequency, the layers inductively couple to one another causing a condition of “pre-loading,” which causes the resultant coil to be less sensitive to loading due to variability among subjects or positioning of the coil. This inductive coupling is controlled by selectively spacing the layers apart from one another, and appropriately tuning each conductor element to a resonance frequency. If there is too much inductive coupling between the layers, then there will be no coupling to the subject and no image can be formed. The amount of coupling between the layers determines the initial impedance pre-loading that moderates the coil element performance over a changing load. This means that stacked coil elements will perform at higher SNR, when used on different subjects of different sizes, or biochemistry, than a single element coil under the same conditions.
The configuration of
The separation distance 5 of
Referring now to
The configuration shown in
Referring now to
It has been found that the coils 10 of the above and following types maintain a higher level of performance over a wider range of load impedance when compared to a single loop coil, as shown in Table 1 below. That is, a single loop coil has a narrower band of SNR performance that rises and falls off rapidly with changes in load impedance (a typical target impedance is 50 ohms). Load impedance helps facilitate the amount of coupling between a coil and a subject. The coil 10 discussed here, having a stacked or twisted configuration, maintains a broader range of SNR performance with changes in load impedance. Since subjects come in all shapes, sizes, densities, and biochemical compositions, changes in load impedance are expected, and may change widely between subjects. A single loop coil that is on resonance, and at peak SNR performance for one subject, may not be at peak performance for another subject. The stacked or twisted configuration of the coil 10 here, will maintain performance closer to the expected peak SNR between a wider range of subjects who offer a broader range of coupling (and load impedance).
Table 1 below shows data where a phantom and coil (either a single or stacked printed circuit board coil) were kept in the same constant configuration, with the coil placed on top of the phantom (“Phantom” loading condition in the table), and various loading containers reflecting different loading conditions (1-4 in the table) were then placed on top of the coil. These different loading conditions (1-4 and phantom only) were used to change the coil circuit impedance which was then measured and recorded in the table. The SNR was measured under each of the loading conditions using the same phantom, coil, and geometry.
SNR and Load Impedances by Coil
(phantom + loader)
In addition to gains in SNR, the coil configurations of the present invention may be utilized for heteronuclear detection. One conductor element can be tuned to detect a first nuclei, and the second conductor element can be tuned to detect a second nuclei for simultaneous detection of both the first and second nuclei signals. Such nuclei may include 1H, 13C, 31P, 23Na, 19F, and the like. Utilizing an appropriate pulse sequence, the heteronuclear coil system may facilitate chemical analysis that provides for chemical discrimination and characterization of compounds present in the subject.
In some array configurations, the elements of an array of twisted coils of the type depicted in
The coil configurations of the present invention may be utilized in MRI, MR spectroscopy, and the like. Additionally, various coil shapes may be utilized, including circular loops, square loops, rectangular loops, fractal shapes, and the like. Accordingly, it will be readily understood by those persons skilled in the art that, in view of the above detailed description of the invention, the present invention is susceptible of broad utility and application. Many adaptations of the present invention other than those herein described, as well as many variations, modifications, and equivalent arrangements will be apparent from or reasonably suggested by the present invention and the above detailed description thereof, without departing from the substance or scope of the present invention.
It should also be readily understood that the figures are meant for illustrative purposes only, and that the twisted or stacked configurations are readily interchangeable for any desired coil configuration, and the twisted and stacked configurations may both be utilized within the same coil array. Also, the present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.
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|Internationale Klassifikation||G01V3/00, G01R33/341, G01R33/3415, G01R33/34|
|Unternehmensklassifikation||G01R33/3415, G01R33/34007, G01R33/34092, G01R33/341|
|7. Nov. 2016||AS||Assignment|
Owner name: MAYO FOUNDATION FOR MEDICAL EDUCATION AND RESEARCH
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:FELMLEE, JOEL P.;REEL/FRAME:040243/0286
Effective date: 20161024